1. Field Of The Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for programming and erasing flash electrically-erasable programmable read only memory (flash EEPROM) arrays.
2. History Of The Prior Art
The use of computers has grown so extensive that the power used by these computers has become significant. In order to reduce the cost of operation as well as the consequent use of energy resources, a substantial move is underway to reduce this power usage. In fact, a major trend in the manufacture of personal computers is to reduce the voltage level required to operate the integrated circuits which are used in the various components of those computers. There is also a simultaneous trend to provide portable computers which function for extended periods. This trend has also has led to attempts to reduce the power used by portable computers.
In order to reduce power consumption and extend battery life, much of the integrated circuitry used in personal computers is being redesigned to run at low voltage levels. This reduces the power usage and allows more components to be placed closer to one another in the circuitry. The circuitry and components used in portable computers are being designed to operate at voltages levels such as 5 volts and 3.3 volts. This helps a great deal to reduce the power needs of such computers.
However, at the same time, the desire to offer more features in portable computers opposes this salutary result. Many of these features require higher voltages to function. For example, one real convenience is the ability to change the basic input/output (BIOS) processes as improvements in a computer or its peripherals occur. Historically, this has been accomplished by removing the electrically programmable read only memory (EPROM) or similar circuitry which provides the read only memory for storing the BIOS processes and replacing it with new circuitry at additional cost. This is a complicated operation beyond the abilities of many computer users. Recently, flash electrically-erasable programmable read only memory (flash EEPROM memory) has been used to store BIOS processes. This memory may be erased and reprogrammed without removing the BIOS circuitry from the computer by running a small update program when the BIOS processes are changed. However, reprogramming flash EEPROM memory requires approximately twelve volts to accomplish effectively; and the lower voltage batteries provided in personal computers are not capable of furnishing this voltage.
Another type of flash EEPROM memory array provides another example of high voltage requirements in portable computers. This type of flash EEPROM memory array provides a new form of long term random access storage. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is described in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. Such an array provides a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is important.
However, these flash EEPROM memory arrays also require much higher voltages for programming and erasing data than can be provided directly by the batteries of low powered portable computers. In other electronic arrangement, charge pump circuits have been used to provide a high voltage from a lower voltage source. However, even though charge pumps have long been available which are capable of providing the voltages necessary for programming and erasing flash EEPROM memory arrays, no arrangement has yet been devised for utilizing these charge pumps to accomplish erasing and programming of flash EEPROM memory arrays using those positive source erase techniques which are used when 12 volts is available from an external source.
There are a number of reasons that this is true. However, the primary reason for the failure is the universal perception that insufficient current can be generated using charge pumps to accomplish the erase process. The positive source method of erasing flash EEPROM memories draws a very substantial amount of current. For example, various source books list the current necessary to erase flash EEPROM memories as 30 milliamperes. For this reason, new negative gate erase techniques which require substantial circuit changes and increases have been devised. We have now discovered that using specially designed charge pumps we are able to generate the amount of current necessary to accomplish positive source erase of flash EEPROM memory arrays. We have also discovered that positive source erase can be accomplished using less current than was expected to be necessary without increasing the required erase time. We have found that charge pumps can be used for providing positive source erase voltages for such arrays. The positive source technique for erasing flash EEPROM memories allows the proven methods of erasing and programming to be used without the need to increase row decode and other circuitry in the manner typical of negative gate erase methods. We have also found that a smaller version of the same charge pump may be used to furnish the voltage with the necessary current at the drain terminal of each memory device during the programming operation.